In the manufacture of semiconductor devices, it is the practice to form uniform thin films of metallic material, usually aluminum or titanium, by deposition on the surface of a silicon semiconductor wafer. The metallic material is deposited on the surface of the wafer and extends downward into etchings or channels formed in the wafer to create electrical connection between various points. Precise control of the process of depositing the metallic films is critical to the operation and quality of the semiconductor device being fabricated. If the metallic layer is deposited insufficiently, or is deposited with contaminants that affect the integrity of the intended electrical connections, then the semiconductor is considered unreliable and, therefore, defective. The losses of production when the metallic layer is not deposited accurately and efficiently are a part of the cost of manufacturing the semiconductor devices, and thus the cost of the devices to consumers increases dramatically.
To produce semiconductor devices on an efficient scale, the industry has used sputtering or Physical Vapor Deposition (PVD) processes. PVD equipment generally includes a source of metallic material which is vaporized in a vacuum. The vaporized metallic particles are forced towards the semiconductor wafer to form the metallic layer thereon.
It has been recognized, however, that the metallic particles formed in the PVD process impact on the wafer at varying angles of incidence. As a result, the metallic layer in the areas of the etchings in the wafer is formed unevenly, leaving a relatively thick lateral layer on the side walls of the etchings and a thin layer at the bottom of the etchings. In fact, the lateral growth on the side walls can be deposited in a manner which occludes the opening of the etching, thereby preventing proper formation of the metallic layer at the bottom of the etching. Occlusion of the opening of the etching also affects later steps in the process involving the deposition of a dielectric material over the wafer.
To prevent these disadvantages, the industry has begun to incorporate collimators into sputtering and PVD equipment. The collimator is typically a metallic structure having a plurality of cells/openings which is positioned between the source of metallic material and the wafer to act as a filter. As metallic particles travel toward the wafer, the particles which are not substantially perpendicular to the wafer impact on the cell walls of the collimator and adhere thereto. Thus, the collimator allows only the metallic particles which follow a substantially perpendicular path to the wafer to pass through. The metallic layer which then forms on the wafer is, therefore, of a generally uniform thickness, and the difficulties associated with lateral growth of the film on the etching sidewalls are eliminated.
Examples of PVD systems, sometimes called "sputtering" systems, and collimator structures for use in those systems are disclosed in U.S. Pat. Nos. 5,330,628 and 5,544,771. As shown in these patents, a collimator is typically formed as a honeycomb structure. The cells of the honeycomb structure may be provided with roughened surfaces to improve the ability of the collimator to capture the metallic particles. Also, the collimator is usually formed with a specific aspect ratio, i.e. the ratio of cell height to cell diameter. The aspect ratio of the collimator determines it effectiveness in filtering non-perpendicular metallic particles. A collimator having a high aspect ratio is more effective in filtering the particles than one with a low aspect ratio. A tradeoff exists, however, since the throughput of the PVD equipment is seriously affected by increasing the aspect ratio of the collimator. As the aspect ratio increases, the throughput of the equipment decreases. Accordingly, the aspect ratios of collimators are particularly chosen to achieve a balance between filtering of particles and acceptable throughput.
However, in the course of carrying out the deposition process, the metallic particles which are filtered by the collimator form a layer on the collimator cells which occludes the cell openings. Specifically, mushroom-shaped growths begin to form on the leading edges of the collimator cells thereby reducing the cell input openings and increasing the aspect ratio of the collimator. Ultimately, the throughput of the equipment is reduced so severely that the collimator must be removed and replaced.
Removing the collimator is not easily or quickly done, and it causes significant downtime for the entire manufacturing process. A major delay is caused because when the collimator is replaced, the vacuum in the vacuum chamber is lost and must be re-established. This process can take up to a day to complete. Also, the collimator must be replaced at least once prior to the point at which the source material is replenished. Accordingly, production downtime is encountered-both for replenishing the source material and replacing the collimator.
There is, therefore, a long felt need in the art for a collimator, especially a collimator for use in PVD processes, which has maximum useful life, whereby production downtime associated with the PVD process is minimized.